A recent increase in the intensity of tropical cyclones in the Arabian Sea may be a side effect of increasing air pollution over the Indian sub-continent, a new multi-institutional study has found.
Traditionally, prevailing wind-shear patterns prevent cyclones in the Arabian Sea from becoming major storms. A paper appearing in the Nov. 3 issue of the journal Nature, however, suggests the weakening of the winds aloft has enabled the formation of stronger cyclones in recent years – including storms in 2007 and 2010 that were the first recorded storms ever to enter the Gulf of Oman.
Until recent decades, cyclones in the Arabian Sea tended to be relatively weak storms. This was attributed to a naturally occurring vertical wind shear over the sea, which tended to weaken and ultimately break up developing storms.
But the study's investigators have found that wind circulation patterns over the Arabian Sea are changing, reducing vertical wind shear and allowing stronger storm development.
"There has been a real uptick in the intensity of tropical cyclones in the Arabian Sea," said the study's lead author, Amato Evan, an environmental scientist in the University of Virginia's College of Arts & Sciences. "We wanted to understand why, and we think we found the reason.
"We are showing that pollution from human activity – as simple as burning wood or driving a vehicle with a diesel engine – can actually change these massive atmospheric phenomena in a significant way. It underscores the importance of getting a handle on emissions in the region."
The weakening wind patterns over the past 30 years have corresponded with a buildup of aerosols in the atmosphere over India, which deflect sunlight from the surface, creating dimming at ground level, the researchers said. The aerosol buildup creates formations known as "atmospheric brown clouds," in which smog from diesel emissions, soot and other by-products of biomass burning accumulates and becomes widespread to a degree significant enough to be a climatic force.
The 1.9-mile-thick brown cloud has been linked to altered rainfall
patterns in South Asia. The brown clouds' large-scale dimming effect
also mitigates the warming of the ocean due to greenhouse gas-driven
Historically, the onset of the summer monsoon season produced strong
winds in the lower and upper atmospheres that traveled in opposite
directions – vertical wind shear – which made formation of cyclones
virtually impossible in July and August. Wind shear has the effect of
cutting off the top of a storm – its exhaust, so to speak – from its
power source near the sea's surface, weakening the storm and causing its
Thus, despite warm sea surface temperatures – which normally contribute
to intensifying cyclones – the Arabian Sea has averaged just two or
three cyclones per year, and those tended to form outside the monsoon
season, when wind shear is diminished.
The researchers identified a trend of increasingly strong cyclones in
the months immediately preceding monsoon season in the region. In June
1998, a significant cyclone struck Gujarat, India, killing nearly 2,900
people. In June 2007, Cyclone Gonu, a category 5 storm with winds in
excess of 156 miles per hour, caused $4 billion in damage when it made
landfall in Iran. In June 2010, a category 4 storm, Cyclone Phet, struck
the coastlines of Pakistan and Oman, causing nearly $2 billion in
"We thought, 'Wow, there's something going on here,' where the strongest
storms ever in the region occurred within a few years of each other,"
Using findings from direct observations and model studies of atmospheric
brown clouds made by Scripps Institution of Oceanography climate and
atmospheric scientist Veerabhadran Ramanathan, a study co-author, Evan
and his colleagues posit that the brown cloud over India disrupts normal
air circulation and inhibits summertime warming of the surface, causing
sea surface temperatures in the northern Arabian Sea to more closely
match cooler temperatures closer to the equator.
"The net effect of this pollution is it cools the water," Evan said.
"You can think of it as being like a sponge that absorbs sunlight that
otherwise would have reached the ocean."
The team also found that the brown clouds changed the circulation of the
atmosphere in such a way as to reduce the climatological vertical wind
shear, thus leading to more intense storms.
"It's a fairly recent change in the circulation, the vertical wind
shear, that seems to be allowing storms to intensify now, rather than
break up as they normally would do," Evan said.
"The only thing that's been systematically changing in this part of the
world is pollution," he added. "There's been a huge growth in pollution
from human activity from the Indian subcontinent over the last 60 or so
years, a six-fold increase in emissions of pollutions like black carbon
Ramanathan said, "This study adds another major dimension to a long list
of negative effects that brown clouds have, including rainfall
reduction, Himalayan glaciers melting, significant crop damages and
deaths of a million or more annually. The one silver lining is that the
atmospheric concentrations of these pollutants can be reduced
drastically and quickly using available technologies."
Evan noted that unless air pollution emitted from the Indian
subcontinent is reduced, it is possible that strong cyclones in the
Arabian Sea may become more commonplace. This has the potential to cause
additional billions of dollars in damage, more loss of life and
disruptions to shipping traffic in the Arabian Sea, the Gulf of Oman and
the Persian Gulf.
The other co-authors of the study are James P. Kossin of the National
Climatic Data Center and the NOAA Cooperative Institute for
Meteorological Satellite Studies and Chul "Eddy" Chung of the Gwangju
Institute of Science and Technology in South Korea.
The National Science Foundation's Division of Atmospheric and Geospace
Sciences funded the study with additional support from the National
Oceanographic and Atmospheric Administration's Climate Program Office.This article was first published on www.newswise.com.